DESCRIPTION: (Applicant's Description) A major advance in the delivery of external beam therapy for prostate cancer has been the development of three-dimensional conformal therapy (3DCRT). Reducing the irradiated normal tissue volume with 3DCRT translates into fewer treatment-related complications. Furthermore, dose escalation studies from multiple institutions have demonstrated that higher doses of external beam radiation achievable with current 3DCRT techniques improve the biochemical cure rate by 20 percent-40 percent in selected prostate cancer patients. The data also suggest that a dose plateau has not yet been reached. Innovative refinements in the conformal technique which can enhance the therapeutic ratio by improving target identification and reducing treatment margins will reduce morbidity at current dose levels and ultimately facilitate further improvement in the cancer control rate in the high risk population. One factor, which limits our ability to take full advantage of closely tailored field arrangements, is the problem of target localization and tracking during treatment. Real time correction of patient positioning and internal organ motion errors by ultrasound image guidance and infrared navigational tracking is a novel approach to achieve treatment margin reduction. An infrared sensor driven navigational system utilized in conjunction with a rigid extracranial body frame immobilizer is under development to link the initial CT simulation image to a real time ultrasound prostate image. Automated image registration of the baseline CT and ultrasound will detect any isocenter misalignment and this information may then be electronically transferred to the treatment couch for automated correction. The purpose of this project is twofold: 1/Perform precision validation studies using a CI standard. 2/Complete a phase III randomized trial to assess the expected reduction in complications achievable with ultrasound-guided precision 3D conformal external beam radiation therapy.